1. Field of Invention
The invention relates to a rotary electric machine. More particularly, the invention relates to a rotary electric machine with a structure design of permanent magnet (PM).
2. Description of Related Art
A preferred material for forming a PM of an electric machine is neodymium iron boron. However, currently, the price of rare earth materials is rising; and therefore in order to reduce the cost, it is expected in this industry to use permanent magnet materials (such as ferrites) with weak magnetic properties but low price to replace the neodymium iron boron. However, since the remanence of the ferrite material is only 0.2-0.44 T, and the maximum magnetic energy product thereof is only 6.4-40 kJ/m3, a simple replacement may cause decrease in output power and efficiency of the electric machine.
Thus, in the background art, methods such as increasing the axial length of a rotor and increasing tangential magnetic steel's width are used for increasing the sectional area of the magnetic steel and improving the output power. However, these methods result in the volume and cost increase of the electric machine. A composite rotor magnetic path structure is also used in the background art. That is, magnetic steels are arranged along both the tangential and the radial directions in the rotor. However, this structure can only use a space within the rotor diameter to place the magnetic steels, and thus the space for receiving the magnetic steels limited.
In view of this, it is a problem desired to be solved by those of relevant skills in this industry regarding how to design an electric machine, in which the air gap flux density is improved, and hence the output power of the electric machine is improved without increasing the total volume thereof.